The trend in modern central processing units (CPUs) and microprocessors is to reduce the power supply operating voltage in order to reduce power consumption and increase the chip density. The power supply reduction may impact other performance considerations as well. Due to design considerations, memory devices, such as dynamic random access memories (DRAMs), may operate at a different supply voltage than the CPU. Some devices may also be required to use more than one power supply voltage so they can signal a CPU and related devices at one voltage and other devices at another voltage. The signals are generally generated by one circuit and are received by another. However, distortion may arise in such signals when the CPU operates at one supply voltage and other circuits operates at a different supply voltage.
One such configuration occurs with modern microprocessors that operate with a nominal power supply voltage of about 2.5V while other circuits in the computer operate with a power supply of about 3.3V. Some of the support integrated circuits, such as clock generators, must operate using both power supplies since they furnish signals to both the microprocessor and other integrated circuits. These support devices must have low distortion between outputs operating at different voltages.
Another common example of multiple power supply voltages occurs in systems with both nominal 5V and nominal 3.3V power supply requirements. Systems with more than two power supplies (e.g. 5V, 3.3V, 2.5 V and/or 1.8V or lower) are also possible.
One conventional approach to reducing distortion is to use differential signaling (ECL, CML, or other different logic techniques). This approach requires moderately complicated drivers and receivers and requires two wires per signal, resulting in significantly more area than a simple implementation. Another approach is to suffer distortion but save size by using a simple transmitter (e.g., a CMOS inverter) operating with one power supply and a receiver (e.g., another CMOS inverter) operating with a second power supply. One such configuration occurs with modern microprocessors that operate at a supply voltage of 2.5. volts, while modern DRAM integrated circuits generally operate at a supply voltage of about 3.3 volts or higher.
Conventional approaches provided compensation for the delay and distortion created when a signal passes through devices operating at different supply voltages. One such approach includes compensating for such delays and distortions by inserting additional delays. Another approach uses cross coupled supply voltages to duplicate the delay or distortion in the different signals to provide a deskewed relationship. Both of the approaches, however, merely compensate for distortion created in a signal which propagates over devices operating at different voltages, rather than reduce it.